in recent years, from the viewpoint of improving fuel economy and enhancing the collision safety of automobiles, high strength steel sheet or aluminum alloy with a high tensile strength has come to be used for frame parts in particular. A high tensile strength material can improve the collision performance without increasing the sheet thickness of the material, so is useful for lightening the weight.
However, due to the higher strength of materials, the warping and torsion of a final shaped article caused by residual stress at the time of press-forming become larger and securing shape precision of the final shaped article becomes an issue.
When shape precision of a final shaped article cannot be secured, a gap is formed with other parts when assembling the article in a vehicle. If the amount of the gap is large, assembly problems result. Accordingly, strict shape precision is demanded from the final shaped article. Further, in the case of a part with a small curvature in a bent part of a final shaped article, that is, a radius of curvature of a bent part is 50 to 2000 mm, a high shape precision is particularly demanded. The shape of the bent part is an arc or a curve with a continuously changing curvature. If there are a plurality of such bent parts at the final shaped article, the warping and torsion in the longitudinal direction of the final shaped article which accompany planar stress of the final shaped article are large. For this reason, it is further difficult to secure precision of the final shaped article.
As a conventional general measure for countering poor shape precision, the method is adopted of using prototypes of the final shaped article or past experience to predict the amount of springback and finishing the shape of the die to a shape different from the shape of the final shaped article so as to satisfy the predetermined dimensions. Further, in recent years, before making prototypes of the final shaped article, springback and other aspects of the press-forming operation have been analyzed based on the final shape using the finite element method so as to make the die and thereby reduce the number of corrections to the die when making prototypes.
However, with designing a die based on trial and error, there was the problem that a long time is taken until devising a shape of a die which sufficiently reduces warping and torsion and until establishing shaping conditions. Further, since trial and error are used to design the die, the cost of die correction soars and therefore there was the problem of reduction of cost of the final shaped article being obstructed.
As a measure for improving the shape precision of the final shaped article, the art of imparting a bead to the final shaped article so as to suppress warping and torsion of the final shaped article (PLT 1) has been disclosed. Further, the art of using the holding surfaces of a die and blank holder to locally press against a blank to form a bead at the blank and thereby increase the tension of the vertical wall part so as to secure the shape precision of the final shaped article (PLT 2) has been disclosed.
The arts which are disclosed in PLT 1 and PLT 2 impart a bead to the final shaped article to improve the product shape to thereby suppress springback. Therefore, the shapes of the final shaped articles to which these can be applied are limited. There is the problem that the arts are not universally applicable.
PLT 3 discloses a press-forming method which improves the shape precision of a press-formed article which has a hat-shaped cross-section which comprises a top sheet part, vertical wall parts, and flange parts. The press-forming method which is described in PLT 3 press-forms a metal sheet into an intermediate shaped article which has tapered parts between the vertical wall parts and flange parts, then again press-forms the tapered parts and flange parts of the intermediate product to obtain the final shaped article.
However, the press-forming method which is disclosed in PLT 3 raises the precision of the angles between the vertical wall parts and the flange parts at the final shaped article and improves the flatness of the flange parts. It does not suppress warping or torsion of the final shaped article as a whole.
PLT 4 discloses a press-forming method which improves the shape precision of a final shaped article which comprises a top sheet part and vertical wall parts and which has a bent part. The press-forming method which is described in PLT 4 bends a metal sheet into an intermediate product which has bending angles of the top sheet part and vertical wall parts giving greater amounts of bending than the final shaped article, then bends it back to the bending angles of the final shaped article.
However, in the press-forming method of PLT 4, when the metal sheet is a soft steel sheet or other metal sheet with a tensile strength which is not that high, the warping or torsion of the final shaped article could be suppressed, but when a high strength steel sheet or other metal sheet with a high tensile strength, warping or torsion of the final shaped article cannot be suppressed. Further, when the final shaped article is provided with flange parts and has a cross-sectional shape of a hat shape, tensile stress easily remains at the flange part at the inside of the bent part, so there is the problem that the warping and torsion of the final shaped article become further larger.